The present invention relates to single crystal casting using a seed crystal in a grain starter cavity of a ceramic shell mold.
Single crystal casting and solidification of molten metal using a ceramic investment shell mold positioned on a chill plate is well known in the art of investment casting to produce such components as single crystal gas turbine engine blades and vanes. In one mode of single crystal casting, the ceramic shell mold includes a lower grain starter cavity having a seed crystal positioned therein and contacting a chill plate. The seed crystal is communicated to an upper mold cavity by a helical passage often referred to as a xe2x80x9cpigtailxe2x80x9d. The seed crystal is placed in the lower starter cavity of a ceramic shell mold that is preformed using the well known lost wax process. When molten metal is introduced into the shell mold, a single crystal is expected to grow epitaxially upwardly from the seed crystal with a preselected crystallographic orientation determined by the seed for propagation through the molten metal in the mold cavity. During single crystal solidification, the melt-filled shell mold is withdrawn from a casting furnace to establish unidirectional heat removal from the molten metal in the mold to produce a solidification front of the single crystal through the molten metal in the mold cavity to form the single crystal casting.
Inconsistencies in the fit of the seed crystal in the lower starter cavity of the preformed ceramic shell mold can result in seepage of molten metal or alloy around the seed between the seed and the adjacent mold wall. When this molten flash material solidifies, it can nucleate multiple grains that grow in the starter cavity instead of the expected single crystal. Such spurious grain nucleation and growth in the starter cavity can result in an increase in scrap castings by virtue of their having a grain orientation outside a preselected specification (e.g. where the [001] crystal axis must be within a certain range of degrees relative to the z axis of the turbine airfoil).
An object of the present invention is to provide a method and apparatus for making a single crystal casting mold that overcome the above problem of spurious grain nucleation and growth that leads to increased scrap castings due to grain misorientation.
In one embodiment, the present invention provides method and apparatus for making single crystal casting mold wherein a preformed metallic seed crystal includes a seed passage in which one end of a fugitive connector member is inserted. The seed crystal comprises a metal or alloy similar or identical in composition to that to be solidified in the mold and having a desired crystal orientation. The other end of the connector member is connected to a fugitive pattern of the article to be cast. A ceramic shell mold is formed about the assembly of the seed crystal, connector member and pattern so as to directly contact the seed crystal to embed it integrally in the mold. The connector member and pattern then are removed, leaving a ceramic shell mold having the seed crystal embedded in the mold at the lower mold end with the seed communicated by a mold passage to a mold cavity having the shape of the article to be cast.
In an illustrative embodiment of the invention, an end of the seed crystal is drilled to form an axially extending passage to receive the end of a fugitive thermoplastic helical or straight connector member in close tolerance fit. A fugitive (e.g. wax) pattern ramp is joined to the other end of the connector member and then to a fugitive (e.g. wax) pattern of the article to be cast. The ceramic shell mold then is invested about the assembly by repeatedly dipping in ceramic slurry, draining excess slurry and stuccoing to build up a desired shell mold wall thickness such that the shell mold is invested directly against a side of the seed crystal to embed it in the mold. The fugitive connector member, ramp and pattern are removed from the invested assembly. A ceramic shell mold remains having the seed crystal embedded therein with the drilled end of the seed communicated by a mold passage to a mold cavity having the shape of the article to be cast. When molten metal or alloy is cast and solidified in the mold, seepage of molten metal or alloy between the seed and the mold wall that occurred heretofore and caused spurious grain nucleation in the starter cavity is essentially eliminated by virtue of the absence of any misfit between the seed and mold wall. That is, the shell mold is formed directly about and in direct contact with the side of the seed during the mold formation process to eliminate misfit therebetween.
The present invention thereby provides a method and apparatus for single crystal casting that substantially reduces scrap castings due to grain misorientation from spurious grain nucleation proximate the seed crystal.
The above objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.